1. Field of the Invention
The invention relates to a circuit arrangement for the simultaneous ignition of a plurality of thyristors connected in series, the control paths of which are connected, by means of a full-wave rectifier, to a secondary winding of a common pulse transformer, the primary winding of which is supplied an alternating current of higher frequency by a control current amplifier.
2. Description of the Prior Art
Such a circuit arrangement is known, for example, from the German Offenlegungsschrift 1,563,395 or from the German Offenlegungsschrift 2,638,637. Therein, the individual thyristors are components of a high-voltage converter. At least one high-voltage cable enclosing the magnet core or cores is used as primary winding for one or more pulse transformers. The secondary windings are connected, by means of full-wave rectifiers, to the control paths of the individual thyristors directly, i.e., without further wiring. By means of a coil between the high-voltage cable to the secondary winding and to rectifier, firing information and the firing energy for each transistor are jointly magnetically transformed.
In this arrangement, much firing energy is required for the ignition of a large number of thyristors. The high firing energy must be taken into account in the design of the control current amplifier. The inductance of the ignition circuit, must be considered because of the insulation spacings required. It is for this reason that a dc-ac converter feeding the high-voltage cable or cables is preferred to the control current amplifier in the known arrangements. This dc-ac converter is operated at a higher frequency, namely in the so-called medium frequency range. The medium frequency range extends essentially from 0.5 to 20 kHz.
Because of the operating mode of the dc-ac converters in common use for this purpose, the curve of the primary current supplied to the primary winding of the pulse transformer is not linear, but sine-shaped, for instance. Each half oscillation of the primary current is followed by a gap where no current flows lasting to the start of the next half oscillation in the opposite direction. The gap may be as wide as a half oscillation. The shape of the current in the secondary winding of the pulse transformer looks the same. After rectification in the full-wave rectifier there appears in the control path of the individual thyristors a gate current consisting of a succession of positive half oscillations with zero current intervals between them. Such a gate current curve may lead to the respective thyristor regaining its blocking ability in the zero current interval, despite the fact that a control signal continues to be applied to the control current amplifier and an ignition is desired.
It is an object of the invention to design a circuit described hereinbefore in such a manner that a sizable gate current flows in the control electrode of each thyristor fed by the common pulse transformer during the zero current gaps in the primary current curve. A non-zero gate current should flow also if the primary current is continuous, such as a sine-shaped curve.